Well logging instruments (or “tools”) are used in wellbores (boreholes) to make, for example, formation evaluation measurements to infer petrophysical properties of the formations surrounding the borehole and the fluids and minerals contained therein. Such well logging instruments may include electromagnetic instruments, nuclear instruments, acoustic instruments, and nuclear magnetic resonance (NMR) instruments, for example.
Well logging instruments may be moved through a wellbore on an armored electrical cable (“wireline”) after the wellbore had been drilled. Such wireline tools are still used extensively. However, the desire for information while drilling the wellbore gave rise to measurement-while-drilling (“MWD”) and logging-while-drilling (“LWD”) instruments, which are generally housed in special customized drill collars forming part of a string of tools called the bottom hole assembly (BHA), used to drill, steer, and log the wellbore. By collecting and processing such information during the drilling process, the wellbore operator can make informed real-time decisions pertaining to the drilling operation, to optimize wellbore trajectory or placement and drilling performance, and/or to acquire log measurements right “behind” the drillbit, in freshly drilled formations prior to borehole and/or formation alteration as a result of the drilling process, and/or for logging “insurance” or “assurance” purposes in boreholes difficult to log using wireline techniques, as the case may be.
MWD instruments may provide drilling parameter information such as drilling mechanics (e.g. axial force applied to a drillbit attached at the bottom of the drill string, also called “weight-on-bit”, torque applied to the drill string, also called “downhole torque”, and downhole fluid pressures either inside or outside the drillstring, sometimes converted into “equivalent fluid density” such as “ECD” and “ESD”), drilling mechanics (e.g. downhole BHA rotations per minute, also called “collar rpm”, drill string shock & vibration, and downhole flowrates, using so called “turbine rpm”), wellbore directional surveys (e.g. wellbore geodetic or geomagnetic direction, and wellbore inclination from vertical), and other information like wellbore temperature. LWD instruments may provide formation evaluation measurements such as formation electrical resistivity or conductivity, complex dielectric permittivity, natural gamma-ray, bulk density and photoelectric factor, thermal or epithermal neutron porosity, thermal neutron capture cross-section (called “SIGMA”), a variety of neutron induced gamma-ray spectra (“inelastic”, or “capture”, or “activation” gamma-ray spectra), sonic transit times or velocities, and NMR relaxation time or diffusion constant distributions. MWD and LWD instruments often have components similar in function to those provided in wireline tools (e.g. transmitting and receiving antennas), but MWD and LWD tools are conventionally constructed to operate in the harsh and hostile downhole drilling environment. The terms MWD and LWD are often used interchangeably, and the use of either term in this disclosure will be understood to include both the collection of formation and wellbore information, as well as data on movement, position, and geometry of the drilling assembly.
Well logging instruments may be used to determine formation volumetrics, that is, to quantify the volumetric fraction, which may be expressed as a percentage, of each constituent present in a given sample of formation being evaluated (these constituents which may be thought of as the “elementary building blocks” of the formation, are also called “end-members”). Formation volumetrics may include the identification of the constituents present, and the assigning of unique responses or “signatures” for each or any of such constituents with-respect-to the different log measurements considered (the signatures of these end-members, are also called “end-points”). Together with a corresponding “earth model”, and the considered measurements “mixing-laws”, the logging instruments' measurements may then be converted into elemental volumetric fractions of one or more constituents. The term “earth model” refers to the geometrical or spatial layout and disposition of various constituents with respect to one another, and the term “mixing-laws” refers to how particular log measurement readings behave or change, with the changing percentages of the constituents present (when the change follows a linear relationship, the mixing-laws are said to be linear).